In our copending application Ser. No. 09/011,013 filed Feb. 2, 1998 (U.S. Pat. No. 5,965,073 of Oct. 12, 1999) and the corresponding German application DE 195 29 135 A1, a process for making dimensionally stable polyester yarn for highly dimensionally stable cords, e.g. for the reinforcement of conveyor belts, seat belts, V belts, hoses and for use as tire cord, has been described.
In this process, the polyester yarn is produced from melt-spun polyester filaments, i.e. polyester filaments which are extruded from a spinneret and gathered or bundled to form the yarn.
In that system, downstream of the spinneret or spinning nozzle, a cooling zone is provided which cools the extruded-melt polyester filaments to at least the solidification temperature, whereupon the polyester filaments can be heated in a counterflow heating zone to a temperature above the glass transition temperature. The polyester filaments can then be subjected to a prestretching in this counterflow heating zone and following the prestretching can be subjected as a yarn to a final stretching operation of an end titer of 0.1 dtex to 7.5 dtex at a temperature of 80.degree. C. to 250.degree. C. with a stretching ratio of 1:1.5 to 1:1.15. The polyester yarn is then wound up with a winding sped of 5000 m/min to 8000 m/min. The end titer or final titer is, of course, the titer following this after-stretching operation.
The process described in this application gives rise to a polyester yarn which over the entire titer range of say 1 dtex to 1100 dtex or greater is highly stable and the method is easy to carry out and convenient.
It has long been recognized that a problem with polyester yarns and even the filaments which go to make up such yarns is the dimensional stability. The dimensional stability DS is generally considered to have a component which can be referred to as the standard elongation, generally given in percent and a component which is temperature based, for example hot air shrinkage (also given in percentage) so that the dimensional stability can be defined as EQU DS=EASL (45N)+HAS (180/190.degree. C).
The expression "EASL" refers to "Elongation at specified load" and "HAS" means "Hot air shrinkage" and these have values for a yarn with a titer of 1100 dtex typically as shown in the following table:
______________________________________ EALS HAS DS ______________________________________ LS (Low .about.18 .ltoreq.3 .about.21 Shrinkage HM (High &gt;3.7 .ltoreq.9 .about.13 Modulus) HMLS (High &gt;3.7 .ltoreq.3 .about.7 Modulus Low Shrinkage ______________________________________
The shrinkage and elongation can only be set at desired values when the dimensional stability DS can be guaranteed. A precondition for such dimensional stability is the crystallinity of the polyester filaments. The higher the crystallinity the more stable are the polyester filaments and hence the yarn made from those polyester filaments. The greater the crystallinity, therefore, the more resistant the yarn is to high temperatures and the smaller is the shrinkage.
In the production of polymer yarns it is known to provide polyethylene terephthalate filaments, a typical polyester, with a crystallinity of 5% to 14% (see PCT/WO 96/20299) or with a crystallinity of 16% to 24% (see PCT/WO 90/04667). However, the polyester yarns produced with such filaments are not fully satisfactory.